Automobile manufacturers and designers have focused on airflow smoothening and efficient sound-insulating methods for reducing noise in the vehicle cabin. Noise sources such as the wind, turbulence, and pressure fluctuations can excite the vehicle body and transmit noise into the car cabin. Other inherent noises of the automotive environment include tire and engine noise, as well as voices of other passengers. Fans and blowers of the heating, ventilation, and air conditioning systems generate noise and also generate local pressure variations in the forced air stream. In some cases the pressure fluctuations produced by an airflow in a mobile vehicle can be 50 dB larger than the desired acoustic response, with a resultant signal to noise ratio at the microphone of −50 dB.
Besides being interested in finding ways to reduce the generation of turbulence, automotive manufacturers recognize the need to reduce the influence of the air pressure fluctuations inside a vehicle cabin upon various audio components such as a microphone of an in-vehicle cellular phone or a voice-recognition system.
In order to improve the signal to noise in a microphones exposed to airflow, some systems increase the active diameter of the diaphragm of the microphone thereby reducing the effects of the pressure fluctuations on the microphones.
Some of the newer automobile microphone systems use electronic processing, multiple microphones, or both to reduce the influence of the pressure fluctuations. These microphones can be located on rear-view mirrors, headliners, overhead consoles or steering columns.
In one example, an in-vehicle microphone system located in an overhead console picks up the driver's voice and uses algorithms in its electronic processing to reduce the influence of pressure fluctuations and reduce background noise. This electronic processing helps improve the transmission quality of the driver's speech.
In another example, a self-contained digital-signal-processing (DSP) microphone system uses a digital microphone array and software algorithms to help reduce voice recognition and audio intelligibility issues common in high noise, automotive environments.
Microphone systems for vehicles would be improved if the influence of airflow within the cabin was reduced and the system did not require multiple microphones or signal-processing software to electronically reduce the influence of pressure fluctuations produced by in-vehicle airflow, thereby increasing the signal-to-noise ratio and improving the fidelity of the microphonic pickups to improve clarity of speech. Therefore, an improved in-vehicle microphone system provides clearer voice recognition, increases speaker intelligibility, enhances other noise reduction techniques, and reduces packaging complexity, circuitry and costs, while reducing the influence of airflow around the vehicle cabin.